Write Blockers: What They Do and What They Demonstrate

What Is a Write Blocker?

A write blocker is a hardware or software tool that prevents any modifying command from reaching a storage device while allowing the forensic examiner to read, access, and image the device's contents. The distinction matters because the central requirement for sound forensic examination is straightforward: the original evidence must not be modified during your work with it. [1] The blocker intercepts commands issued by the host computer or forensic tool, permits read operations to proceed, and blocks write operations before they reach the protected drive.

For practitioners, this means a write blocker sits between your examination workstation and the evidence drive. A command from your forensic software arrives at the blocker; the blocker evaluates whether the command would modify data; if it would, the blocker returns a status to the software indicating the operation failed—the command never reaches the drive. [2] If the command is a read operation, it passes through unchanged. The result is that you can image, examine, and analyze the drive while maintaining evidence integrity.

Hardware and Software Write Blockers

Write blocking can be implemented in two ways: as a physical hardware device or as a software application running on the examination computer. [1]

A hardware write blocker is a standalone device that connects the evidence drive to your workstation via USB, FireWire, SATA, or another interface. It examines every command at the hardware level before the drive ever receives it. The advantage is that the blocking occurs outside the operating system and forensic software—lower in the chain—and therefore does not depend on the reliability of either. A hardware blocker creates physical isolation between the evidence and any software on your computer that might inadvertently modify the drive. This is particularly important because forensic examiners cannot always predict what background processes, automatic indexing, or network services their examination workstation might attempt to perform against a newly connected drive.

A software write blocker operates within the operating system itself, intercepting write commands before they can reach the drive through the OS kernel. The software blocker is less expensive and more portable than hardware alternatives, but it provides isolation only at the software level; it relies on the integrity and correct operation of the operating system itself. [2]

For most forensic practitioners working with seized hard drives, SSDs, and external media, a hardware write blocker remains the standard practice because it provides the most reliable independence from the examination environment.

Testing and Validation: The NIST Standard

The National Institute of Standards and Technology has established specifications and test procedures for write blockers through its Computer Forensic Tool Testing (CFTT) program. The goal of the program is to establish a methodology for testing computer forensic tools by developing general tool specifications, test procedures, and test sets. [2]

The Hardware Write Blocker (HWB) specification defines functional requirements for hardware write blocker devices. [2] Those requirements are then used to derive test assertions and a test plan that measure whether a given device actually does what it claims: block write operations and permit read operations. [4]

Testing a write blocker involves two key elements. First, testers attempt to write to the protected drive using a variety of write commands and verify that the blocker prevents the write from succeeding. Second, testers verify that the blocker does not interfere with reading data from the drive—that read operations complete successfully and match the expected data. [4] A blocker that blocks all writes but also accidentally prevents all reads would be useless for forensic imaging.

The NIST CFTT database publishes test results for commercially available write blockers. For example, the Digital Intelligence Ultrablock SATA, when tested against the Hardware Write Blocker Assertions and Test Plan, showed that it "always blocked any commands that would have changed user or operating system data stored on a protected drive and also always allowed commands to read the protected drive in all test cases run." [8] This kind of documented test result provides objective evidence that the device performs as specified under controlled conditions.

The Scientific Working Group on Digital Evidence (SWGDE) classifies write blockers as critical forensic tools that are preservation tools—devices that prevent changes to evidence or other data. [6] Because write blockers operate on original evidence, their proper function is foundational to the defensibility of any examination that follows.

What Write Blockers Demonstrate in the Chain of Custody

A write blocker does not prove that data was not altered. Rather, it demonstrates that you took a deliberate step to prevent alteration. This distinction is important for testimony and documentation.

When you use a write blocker and document that fact in your examination report or acquisition log, you are showing that:

• You isolated the original evidence from the examination workstation using a recognized tool designed for that purpose;
• You did not rely on the operating system or forensic software alone to protect the drive from accidental modification;
• The tool itself has been tested and validated against published specifications.

This showing supports the chain-of-custody narrative. It answers a defense challenge that might otherwise arise: "How do we know you didn't alter this drive while working on it?" Your answer is: "I used a tested hardware write blocker, which is documented here, and here are the test results showing it blocks write commands." This is not proof of innocence—you could theoretically have removed the blocker, altered the drive, and reattached it—but it is evidence of a systematic precaution.

The significance becomes clearer when you consider the alternative: working without a blocker means you must argue that although the OS was running on the workstation, although network services were active, although automatic indexing or backup software might run, none of these altered the drive. A write blocker shifts the burden from explanation to prevention.

Documentation in Acquisition Logs and Reports

Forensic best practice requires recording the make, model, and serial number of the write blocker used, along with when it was used and on which drive. This information should appear in the acquisition log or examination report in the same way you would document the forensic imaging tool, the examiner's name, and the date and time of the examination.

The NIST guidance emphasizes that test results and tool specifications provide information "for toolmakers to improve tools, for users to make informed choices about acquiring and using computer forensics tools, and for interested parties to understand the tools capabilities." [2] This principle applies equally to documentation: you document the blocker so that another examiner, a defense counsel, or a court can understand exactly what protection was in place and can verify the tool's specifications if necessary.

Beyond the existence of the blocker, documentation should also capture:

• The serial number of the evidence drive and the blocker (to prove the same devices are discussed throughout);
• Whether the blocker was tested or verified before use (many examiners perform a preliminary test write to confirm the blocker is functioning);
• Any unusual events during the blocking or imaging process.

This documentation makes the blocker a traceable, verifiable element of your chain of custody.

Limitations: Phones, Cloud Storage, and Beyond

The NIST and SWGDE standards address traditional storage devices—hard drives, SSDs, USB thumb drives, optical media, and removable storage. Write blockers are not a complete answer for all digital evidence scenarios.

Mobile devices. A smartphone or tablet cannot be connected to a traditional write blocker. The device's operating system controls access to its storage and does not expose that storage as a standard block device to external hardware. Write blockers for mobile devices do not exist because the architecture of the device does not permit the interposition of a read-only intermediary. Instead, mobile forensics relies on other techniques: logical acquisition (imaging files accessible through the device's interface), physical acquisition (imaging the raw storage using specialized software and hardware designed for that device model), and documentation of the examiner's actions within forensic acquisition software. The burden of showing non-alteration falls on the software's integrity controls, the examiner's training, and the device's own log of access events.

Cloud accounts and remote storage. Write blockers cannot be placed between an examiner and cloud storage because the examiner never has direct access to the underlying storage device. Instead, the connection is through the cloud provider's application interface (API). Preservation of cloud evidence depends on authentication (proving access was authorized), logging (documenting what was accessed and when), and the provider's own controls over data modification. An examiner typically cannot create a forensic image of a cloud account in the same way as a local drive; instead, the examiner downloads accessible files through the provider's interface and documents the access and download process.

Encrypted drives. A write blocker prevents write commands but does not decrypt the drive. If the evidence is an encrypted drive and you do not have the password, a write blocker will protect it from accidental modification—but you will not be able to read it either. In this case, the blocker demonstrates that you took steps not to alter the physical media while you worked on obtaining the encryption key or attempting decryption.

These limitations do not diminish the role of write blockers for traditional storage devices. Rather, they highlight that a complete forensic practice requires different approaches for different media types, and no single tool solves all preservation problems.

The Practical Standard

A write blocker is expected practice in forensic imaging of seized computers and storage devices. Using one and documenting its use demonstrates methodical evidence handling and reduces vulnerability to claims of alteration. Because the tool is tested, validated, and its testing results are publicly available, it also provides an objective answer to a skeptical question: "What did you do to protect this evidence from being changed?" The answer—a tested hardware write blocker—is specific, defensible, and rooted in published standards.

This is not the same as saying a write blocker is mandatory in every jurisdiction or every case, or that its absence renders evidence inadmissible. Admissibility is decided by the court. It is saying that using a write blocker is the recognized best practice in the field, that it serves a clear protective function, and that documenting its use strengthens the integrity narrative in your case.